BSTJ 10: 3. July 1931: New Standard Specifications for Wood Poles. (Jones, R.L.)

New Standard Specifications for "Wood Poles

By R. L. JONES*

This paper summarizes the work of the Sectional Committee on Wood
Poles of the American Standards Association covering the preparation of
specifications for northern white cedar, western red cedar, chestnut and
southern pine poles. The major problems underlying the development
of standard ultimate fiber stresses, standard dimension tables and practical
knot limitations are discussed and illustrated by supporting tables or
figures. Graphical charts comparing the old and the new dimensional
classifications are described. The main points relating to the material
requirements for the four pole species are outlined briefly.

REPRESENTATIVES of communication, power and light, and
transportation utilities, of producers, and of public and general
interests have cooperated in the preparation of the new uniform stand-
ard specifications for wood poles that were recently approved by the
American Standards Association. 1 The new specifications cover di-
mensions and material requirements for northern white cedar, western
red cedar, chestnut and southern pine poles, but rules for preservative
treatment are not included. Specifications for lodgepole pine and
Douglas fir poles are in preparation.

Pole specifications deal with natural rather than fabricated prod-
ucts. Heretofore, the larger utilities have purchased poles of the
various species under specifications that have grown up more or less
independently. Confusing differences in material requirements and
in the dimensional tables have resulted. Economic production and
utilization require the arrangement of the natural cut of pole timbers
into groups defined either by top diameters and lengths, or by classes
in which circumferences at the top and butt are specified in addition
to length. The letter designations, such as A, B, and C, that have
been applied to these classes, have had no common meaning. A pole
of a given length and class of one species has not generally been equiva-
lent in strength rating to one of the same length and class of another
species; and in most cases, the longer poles of a given class have not
had the same strength rating as the shorter poles of the same class.

It is perhaps quite obvious that before rational improvement could
be made in the system of dimensional classification, it was necessary
to create a foundation for comparison of the strength of the different

* Chairman, Sectional Committee on Wood Poles, American Standards Asso-
ciation.

1 These specifications were approved on June 20, 1931.

514

NEW STANDARD SPECIFICATIONS FOR WOOD POLES 515

species. For illustrative purposes a summary of part of the test re-
sults used in arriving at fiber stress values is shown in Table 1. A
detailed study of the results of these tests and of other tests made on
full length poles and on small clear specimens of wood of the species

TABLE 1

Summary of Statistical Analysis of Modulus of Rupture Values Obtained
from Tests on Full Size Poles

Northern

Western

Southern

Modulus of

White

Red

Chestr <

Pine

Rupture

Cedar

Cedar

(Creosoted)

Pounds per

Square Inch

No.

Per cent

No.

Per cent

No.

Per cent

No.

Percent

2000-2499

2

3.57

2500-2999

13

23.21

1

0.66

3000-3499

11

19.64

4

2.65

3500-3999

14

25.00

5

3.31

1

1.02

4000-4499

8

14.29

10

6.62

4

4.08

4500^999

1

1.79

21

13.91

7

7.14

1

0.83

5000-5499

5

8.93

21

13.91

8

8.16

1

0.83

5500-5999

2

3.57

18

11.92

14

14.29

6

4.96

6000-6499

21

13.91

15

15.31

4

3.31

6500-6999

25

16.55

11

11.22

12

9.92

7000-7499

16

10.60

14

14.29

28

23.12

7500-7999

7

4.64

13

13.27

10

8.26

8000-8499

1

0.66

7

7.14

15

12.40

8500-8999

1

0.66

3

3.06

15

12.40

9000-9499

12

9.92

9500-9999

1

1.02

5

4.13

10000-10499

5

4.13

10500-10999

6

4.96

11000-11499

1

0.83

Total Xo

56

151

98

121

Average

3670

5813

6536

8039

Standard deviation. .

860-

1184

1223

1348

Coefficient of varia-

tion (percent) ....

23.43

20.39

18.71

16.77

2 Uncorrected for sample size.

under investigation led to the recommendation of the following figures
as standard ultimate fiber stresses:

Northern white cedar 3600 lbs. per sq. in.

Western red cedar 5600 " " " "

Chestnut 6000 " " " "

Southern yellow pine (creosotedj 7400 " " " "

The fiber stress for a given species finds application in pole line
engineering through the conversion of the stress value into terms of
moment of resistance, usually at the ground line. The poles act as a
series of supports for the wires. With this in mind one of the studies
conducted in connection with the application of the new fiber stresses,

516 BELL SYSTEM TECHNICAL JOURNAL

which is cited here by way of illustration, was directed toward an
analysis of the variation in size and variation in modulus of rupture
that might be expected to affect the average ground line moment of
resistance of random 3 pole groups. Approximately 400 creosoted
southern pine and 500 western red cedar, class 3, thirty foot (see
Table 2) poles were used in this particular study. It was found that
in more than 95 per cent, of the cases the average moment of resistance
of such 3-pole groups was higher than the minimum calculated for
the given class and length. The result is considered reasonably rep-
resentative of what would be found in a similar study of other sizes.
It may be concluded that with the new standard fiber stress values
as a basis practically all parts of a line when new should be equal to
or better than the strength rating for the specified minimum of the
class of poles used ; and that when the reduced loads under the con-
ditions usually obtaining in the higher grades of construction are
considered, the bending moment developed at the ground line should
rarely, if ever, approach the actual moment of resistance.

Since the standard ultimate fiber stresses are based upon tests of
representative poles, they are believed to be satisfactory for all ordi-
nary purposes. They are directly applicable in the engineering of pole
lines without further adjustment or compensation for knots, variation
in moisture content, or density of wood. In any case, the question of
density classification may be limited for practical purposes to southern
pine poles; and studies of current production show that approximately
75 per cent of such poles passing through the producers' yards could
be classified as dense. The creosoting process seems to reduce the
variation found in the modulus of rupture values of untreated poles.
The comparatively low coefficient of variation of creosoted southern
pine shown in Table 1 indicates that for general purposes an attempt
to classify pine poles according to density is an unnecessary refinement.

With the standard fiber stresses as bases, dimension tables for the
four species were developed in accordance with the following prin-
ciples:

(a) The tables should specify dimensions in terms of circumference

in inches at the top, and circumference in inches at six feet
from the butt for poles of the respective lengths and classes
except for three classes with "no butt requirement."

(b) All poles of the same length and class should have, when new,

approximately equal strength, or in more precise terms, equal
moments of resistance at the ground line.

(c) All poles of different lengths within the same class should be of

suitable sizes to withstand approximately the same breaking

NEW STANDARD SPECIFICATIONS FOR WOOD POLES 517

load assuming that the load is applied two feet from the top
and that the break would occur at the ground line.
(d) The classes from the lowest to the highest should be arranged in
approximate geometric progression, the increments in breaking
load between classes being about 25 per cent.

Item "d" is in accord with the preferred number principle, and the
increments chosen provide the lowest number of classes that are re-
quired in service.

Tables of ten classes for each species, as shown in Table 2, have
been made a part of the standard specifications. Classes 8, 9, and 10,
defined simply by minimum top circumferences, have been provided
to cover poles purchased on a top size basis or for rural or other
lightly-loaded lines. Classes 1 to 7, defined primarily by their cir-
cumferences at six feet from the butt, have been designed to meet the
following breaking loads in pounds, assuming the conditions of item (c) :

Class 1—4500 Class 5—1900

Class 2—3700 Class 6—1500

Class 3—3000 Class 7—1200
Class 4—2400

The required circumferences at the ground line for the respective
species were calculated by means of the formula Mr = .000264f C 3 ,
which is the well-known flexure formula applied to a cantilever beam
of circular cross section, and reduced to foot pound units. The ground
line circumferences thus obtained were converted into circumferences
at six feet from the butt by means of approximate average taper values
for the respective species.

The breaking loads are ratings for the minimum size pole for the
given length and class based on the standard ultimate fiber stress for
the species. The average pole of a given class will usually be consid-
erably stronger than the class rating. The choice of sizes provided
in the tables is sufficiently extensive to enable the engineer to make
an economical selection of poles to meet specific requirements after the
load conditions of the line have been determined.

Graphical charts have been prepared which show the relation be-
tween the dimension tables of some current specifications and the new
standards. These charts should be of material assistance to suppliers
and consumers who wish to compare the old with the new for inventory
or record purposes. Representative blocks from the charts appear in
Fig. 1. Comparisons for all lengths and classes may be found in the
complete charts that are obtainable from the American Standards
Association.

518

BELL SYSTEM TECHNICAL JOURNAL

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BELL SYSTEM TECHNICAL JOURNAL

Employment of the new standard ultimate fiber stresses of wood
poles is provided for under rule 261-4-c of the National Safety Code.
With the revisions necessitated by their adoption, Table 20 of the
Code will appear as indicated in Table 3.

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MINIMUM CIRCUMFERENCE AT 6

pi g< l— Representative block from the graphical charts for southern yellow pine —
current dimensions compared with the new standard tables.

The material requirements of the several specifications cover shape,
and straightness of grain, and limit or prohibit such defects as knots,
checks, insect damage and decay. Without detailed reference to what
might be called the appearance requirements, it may be said that the

NEW STANDARD SPECIFICATIONS FOR WOOD POLES 521

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522

BELL SYSTEM TECHNICAL JOURNAL

specifications define poles of a quality that the major utilities have
found to be satisfactory. Departures from straightness are held within
practical limits for ordinary use.

Decay and the presence of wood-rotting fungi are generally pro-
hibited. Minor exceptions are made with respect to the butts of the
cedars, which are usually treated with creosote. The question of
including poles cut from sound dead trees received careful considera-
tion. Blighted chestnut is acceptable with certain restrictions, but
in the case of the other three species poles from live timber are speci-
fied. While it might appear economical to salvage and use all sound
dead trees standing in the woods, practical opinion at present strongly
favors eliminating dead timber as a source of pole material because
of the extra costs involved in handling and inspection.

It has proved impracticable to limit checks in a precise manner.
Checks or lengthwise separations of the wood fibers vary so much
with the age, seasoning, and moisture content of the pole that although
definite limitation seemed desirable the compromise finally adopted
is one which simply prohibits injurious checks. Practically the matter
is left to the judgment of the supplier and consumer concerned.

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1 2 3 4 5

DIAMETERS, IN INCHES.OF
MAXIMUM SINGLE KNOTS

I 23456789 10

SUMS OF DIAMETERS, IN INCHES.OF MAXIMUM

GROUPS OF KNOTS IN 12 INCH SECTIONS

CURVE A
CURVE B

POLES UP TO AND INCLUDING 45 FEET IN LENGTH
POLES 50 FEET AND LONGER

NOTE: "PER CENT OF POLES" REFERS TO THE PER CENT OF POLES HAVING SINGLE

KNOTS OR GROUPS OF KNOTS SMALLER THAN THE SIZES INDICATED ON THE BASE
LINE. FOR EXAMPLE, 58 PER CENT OF THE POLES 50 FEET AND LONGER HAVE
MAXIMUM SINGLE KNOTS SMALLER THAN 3 INCHES IN DIAMETER

Fig. 2 — Knot sizes in southern pine poles.

The limitation of knots was a matter of special study. Previous
specifications were at variance and data were lacking to establish
acceptable limits. Measurements of knots larger than one half inch
were therefore made on representative poles of the four species. The
size and location of about twenty-three thousand knots in some 567

NEW STANDARD SPECIFICATIONS FOR WOOD POLES

523

poles were tabulated, and as might have been anticipated, the occur-
rences of large knots or large groups of knots were found to increase
with the length of pole. This led to a division of the data into a
group for short poles and one for long poles of each species. Figure 2,
for southern pine, is a typical illustration of the curves drawn from the
data. It shows, first, the per cent of poles that have single knots of
the given diameters, (A) for poles up to 45 feet long, and (B) for poles
50 feet and longer; and second, the per cent of poles having groups of
knots with the indicated sums of diameters in any 12 inch section,
separately plotted for the same two cases. The limits set by this
study for single knots and for groups of knots in a twelve inch section
are shown in Table 4.

TABLE 4
Specification Limits for Knots

Southern
Pine

Chestnut

Western
Red
Cedar

Northern
White
Cedar

Single Knots

Poles 45 ft. and under *

Poles 50 ft. and over * .
Group of Knots
(12 in. Sections)

Poles 45 ft. and under. .

Poles 50 ft. and over. . .

(Diameter — Inches)

3 and 4f
5

Ki

4
5.5

(Sum of Diameters — Inches)

7 10

9 10

2.5
4.5

11

* Except for Northern White Cedar where the length division points are 35 ft.
and 40 ft.

f 3 inches for Classes 4 to 10; 4 inches for Classes 1 to 3.

The standards referred to above which have been prepared and
approved under the procedure of the American Standards Association
are nine in number. One prescribes the ultimate fiber stresses for
poles of northern white cedar, western red cedar, chestnut and south-
ern pine, and four prescribe the dimensional classifications for each of
the above species according to lengths and circumferences as shown
in Table 2. These five are American Standards. The situation with
respect to checks and dead timber led to recommending the remaining
four specifications covering material requirements as American Tenta-
tive Standards. They are the first American standards for wood poles
and their adoption on the sound basis outlined marks an important
step toward simplified practice in an essential public utility com-
modity.

The application of the results of the work, as is true of other well-
conceived standardization projects, should yield many engineering

524 BELL SYSTEM TECHNICAL JOURNAL

and economic advantages. The specifications will facilitate good en-
gineering and help to clarify questions bearing on the joint use of
poles. No attempt has been made to evaluate the economic savings,
but, in the long run, bringing substantially all production and utili-
zation together upon the basis of rational uniform sizes and specifica-
tions may be expected to produce economies and benefits in which all
concerned should share.